Study Notes on Genetic Variation and Natural Selection

Genetic Variation and Geographic Influence

• Genetic variation occurs at the population level and can be influenced by geography and location.
• Populations can be separated or have overlapping ranges, affecting allelic exchange.
• Alleles may be passed along a gradient, influenced by environmental factors (e.g., temperature or salinity).

Cline and Natural Selection

• A cline is a graded change in a character across a geographic axis, often due to environmental variations.
• Natural selection leads to differential survival, influencing allele frequency along a gradient.
• Example: A fish called mummichog shows high frequencies of a cold-water enzyme allele (LDHB lowercase b) in northern populations, favoring survival in cold conditions.

Genetic Drift

• Chance events can lead to genetic drift, where allele frequencies change randomly, impacting genetic variation.
• Silent mutations can influence the gene pool without affecting fitness.

Examples of Allele Frequency and Environmental Adaptation

• In northern populations, the LDHB allele is prevalent (e.g., 100% in Maine), while in southern populations (Georgia), it is rare (e.g., 10%):
  • This difference is driven by natural selection based on water temperature.
• Another example involves mussels in varying salinity; alleles for osmoregulation (LAP 94 allele) are more frequent in full salinity environments.